1. Field of the Invention
The present invention relates to an image forming apparatus and method, and more particularly, to the structure of a recording head suitable for an inkjet recording apparatus forming color images by using inks of a plurality of colors, and to recording control technology for same.
2. Description of the Related Art
When printing a color image, an inkjet printer uses inks of at least three colors, cyan (C), magenta (M), and yellow (Y), and furthermore, it may also form images using black (Bk), light cyan (LC), light magenta (LM), dark yellow (DY) and a special color (SPC), and the like.
In general, there are many examples of printers corresponding to print output of high quality (photographic standard quality) which use six or more colors of ink. In inkjet printers of this kind, generally, the nozzle density in the head is set to the same density for each of the colors. Examples are known wherein document printing speed is emphasized, and Bk nozzles only are provided in greater number and higher density than the other colors, but in this case, all of the colors other than Bk are set to the same nozzle density as each other. In other words, generally, as the number of colors becomes greater, so the number of nozzles increases, accordingly.
Furthermore, in a conventional inkjet printer, in order to shorten the printing time, the time interval between ink discharges has been shortened (the discharge frequency has been increased), and the number of ink discharge nozzles in the recording head has been increased. Increase in the discharge frequency has been achieved either by raising the upper limit of the response frequency of the discharge mechanism (the pressurizing devices, such as a piezo element, or the heater), or by replenishing ink more quickly after ink discharge. Furthermore, increasing the number of discharge nozzles is achieved by improving the head processing and fabrication technology, and increasing miniaturization and density, and even in an inexpensive inkjet printer, the overall number of nozzles can be several thousand.
Due to improvements of these kinds, it has been possible to shorten the printing time, but on the other hand, the following types of problems have arisen. More specifically, the increase in the number of nozzles described above leads to problems in that, in addition to raising the cost of the device, the increase in the total number of nozzles, and the fact that the total length of the flow passages inside the head for supplying ink to these respective nozzles becomes longer, give rise to an increased possibility of an ink discharge problem occurring in the head.
This is not limited to an increased probability of simple breakdowns, but rather means that there is an increased possibility of problems which are intrinsic to inkjet systems, such as air bubbles becoming trapped inside the ink flow passages and it becoming impossible to perform normal discharge, or the ink viscosity rising in the vicinity of the nozzles and causing discharge failures.
More particularly, in a single-pass type inkjet printer, which, unlike a shuttle scan type printer for printing by scanning an inkjet head back and forth, has a fixed head of a length not shorter than the print image and performs printing by conveying printing paper in a direction orthogonal to the longitudinal direction of the head, the number of nozzles per ink color may exceed 10,000, and hence the issue of increased possibility of problems such as those described above is very serious indeed.
In order to deal with the issue of problems of this kind, although it runs counter to improvements aimed at enhancing image quality, if the number of nozzles could be reduced, then the possibility of problems occurring can also be reduced.
In relation to reducing the number of nozzles, Japanese Patent Application Publication No. 2000-94717 discloses technology wherein, in an inkjet printer using a head having ink discharge nozzles arranged in a staggered matrix, a structure is adopted whereby black, cyan, magenta and yellow (Bk, C, M and Y) inks, a diluting ink, and a cleaning solution can be supplied to any desired subsidiary tank of the head, and nozzle rows are allocated in a two-row arrangement for magenta and cyan, and nozzle rows are allocated in a one-row arrangement for yellow and black, whereby the number of nozzles for the two colors, yellow and black, is half the number of nozzles for cyan and magenta.
In Japanese Patent Application Publication No. 2000-94717, an embodiment is disclosed wherein yellow and black are set to 300 dpi and cyan and magenta are set to 600 dpi; however, there is no disclosure regarding the method for ejecting droplets of yellow ink in order to achieve a suitable density which balances with the other inks, or regarding the dot size and ink density of the yellow ink, in the case where the number of nozzles for yellow ink is fewer than those for other inks.
Moreover, Japanese Patent Application Publication No. 2003-127438, discloses technology for reducing the resolution of the yellow color only and technology for reducing the resolution of colors having a smaller number of nozzles, if different numbers of nozzles are provided for each color.
In the technology proposed in Japanese Patent Application Publication No. 2003-127438, even in a head composition in which the number of nozzles of other colors of ink is one half that of the number for black, when forming color images, the aim is, however, to increase speed by reducing the image resolution and reducing the number of scan operations by one half, and the technology indicates how to print the data corresponding to two pixels of the color ink, in one pixel, and how to assign print data for light type inks and for normal inks in divergent nozzle positions based on a staggered matrix arrangement, but it does not resolve the problem of reducing the possibility of the occurrence of problems due to the increase in the number of nozzles as described above.